Try this in a child theme of twenty eleven, just copy the twenty elevent style.css to the child theme directory:

here are the changes in the CSS, you’ll have to find the appropriate places in the CSS (or use the !important tag in a blank style.css). These are not intended for direct cut and paste, but they are the relevant CSS to modify to get the nav to align center.

You may want to target your CSS more directly than just #access { text-align:left; } for instance #access ul ul a { text-align:left; }

You definitely want to be pretty comfortable with CSS before taking my hints, they are more like shortcuts. I’m afraid I can’t troubleshoot CSS details, but your best friend is FireBug for testing this stuff: